This invention relates to a fracturing method for improving hydrocarbon recovery. More particularly, the invention is a method of fracturing an underground formation by injecting a mixture of carbon dioxide and a polar alcohol or polar glycol.
Underground formations are frequently fractured to stimulate the production of oil and gas. Fracturing may occur by injecting a fracturing fluid of liquid, gas or 2-phase fluid down a wellbore at sufficient pressure and flow rate to fracture the underground formation. Optionally, a proppant material such as sand, fine gravel, sintered bauxite, glass beads or the like can be introduced into the fractures to keep them open. The propped fracture provides larger flow channels through which an increased quantity of hydrocarbons can flow. A proppant material may also be carried into the formation by the fracturing fluid.
One fracturing technique has been to utilize a liquified, normally gaseous fluid such as carbon dioxide. U.S. Pat. No. 3,195,634 discloses a method of fracturing an underground formation with a liquid mixture of carbon dioxide and water. This fracturing fluid optionally includes a gelling agent and proppant material. Upon pressure release at the wellhead, the liquid carbon dioxide vaporizes and flows from the formation.
Liquid carbon dioxide fracturing has been performed in the field on numerous wells. The advantages and limitations of carbon dioxide fracturing are discussed in Sinal, M. L. et al., "Liquid CO.sub.2 Fracturing: Advantages and Limitations," The Journal of Canadian Petroleum Technology, Sept.-Oct. 1987, pp. 26-30. The article states that liquid CO.sub.2 fracture treatments have been primarily used on gas wells as opposed to oil wells since gas formations can take maximum advantage of the smaller fractures created by carbon dioxide fracturing. Carbon dioxide is injected directly into the formation as a liquid and pumped with conventional frac equipment. In order to maintain adequate viscosity to generate a fracture with sufficient for sand injection, it is believed that carbon dioxide must remain in its liquid phase. This requires the bottom-hole temperature to be reduced to 31.degree. C. or less, the critical temperature of carbon dioxide.
The chief advantages of liquid carbon dioxide fracture treatments are the elimination of formation damage associated with conventional frac fluids. Since carbon dioxide exists in its vapor phase at reservoir temperatures and pressures, carbon dioxide fracturing results in zero residual frac fluid saturation. In gas reservoirs, this completely eliminates any relative permeability or capillary pressure damage around the fracture face.
A second major advantage is that the evaluation of a fractured zones' potential is almost immediate because of rapid clean-up. The substantial energy provided carbon dioxide results in the elimination of all residual liquid left in the formation from the frac fluid.
Third, fracturing with carbon dioxide is economical. Costs for frac fluid clean-up and associated rig time is considerably less than with conventional frac fluids. Swabbing of the well is completely eliminated as a post-frac treatment, and no disposal of recovered frac fluid is required.
The main disadvantage of carbon dioxide fracturing is the low viscosity of the fluid. Proppant weight and concentration are significantly less than that which can be carried by a conventional frac fluid. Fluid leak-off is also high due to the low viscosity. Consequently, carbon dioxide fracturing is not applicable to high permeability reservoirs.
The New Mexico Petroleum Recovery Research Center has performed tests with the use of high molecular weight polymers for increasing carbon dioxide viscosity. Extensive testing on a number of commercially available polymers has failed to find a solution. High molecular weight polymers do not have a sufficient solubility to alter carbon dioxide viscosity. These tests have been reported in Heller, J. P., Dandge, D. K., Card, R. G., and Donaruma, L. G., "Direct Thickeners for Mobility Control of CO.sub.2 Floods," SPE Journal, October 1985.
Two publications have noted relatively large increases in carbon dioxide densities with the addition of relatively low molecular weight compounds. These publications did not, however, mention viscosity. See Paulaitis, M. E., Penninger, J. M. L., Gray, Jr., R. D., and Davidson, P., Chemical Engineering at Supercritical Fluid Conditions, Ann Arbor Science, pg. 31-80 (1983); and Snedaker, R. A., Ph.D. Thesis entitled "Phase Equilibrium In Systems with Supercritical Carbon Dioxide," Princeton University (1957).
The ability to predict the viscosity of a carbon dioxide and decane mixture by two correlations between density and viscosity was compared with actual measurements in Cullick, A. S. and Mathis, M. L., Journal of Chemical Engineering Data, Vol. 29, pg. 393-6, (1984).